They said those who most often are anxious or depressed were 40 times more likely to develop mild cognitive impairment, a form of memory loss that is often a transitional stage between normal aging and dementia.

"Not only are these individuals losing cognition, but they are showing many of the changes in the brain that are associated with Alzheimer's disease," said Robert Wilson of Rush University Medical Center in Chicago.

In prior studies, Wilson found that people who are more prone to distress are more likely to develop Alzheimer's disease than their more carefree counterparts.

"We now see that chronic distress is related to the first clinical manifestation of Alzheimer's disease," he said.

People with mild cognitive impairment have some trouble remembering things, but they do not have significant disability. Not all people with this problem develop Alzheimer's disease, a much more serious impairment, but about 10 to 15 percent do, according to the Alzheimer's Society.

"What we're measuring is a personality trait that we all have to greater or lesser degree. We all experience anxiety and periodic depression. This trait helps identify people for whom that is more characteristic than others," said Wilson, whose study appears in the journal Neurology.

"This isn't a measure of stress, but of the response to stress," he said.

The latest research suggests that chronic stress may harm parts of the brain responsible for responding to stress - an area that is also associated with memory, he said.